The act of stretching one's scrotal skin out with two hands - using thumb and forefinger - parallel to a primary surface (i.e. a bed or floor) until smooth, with the penis resting on top, utilizing the grip and taut skin to move the scrotum up and down at various speeds to have the penis bounce off of it continuously.

The process of fingering a girl when she still has her panties on. You would push on the outside of the panties, pushing them into the womens vagina. It is in-titled the Trampoline because the panties push back just like a trampoline would. Trampolining a girl is not consider a good thing, and you should make fun of a friend if that is all he/she has done, however it is better then getting nothing.

"Dude did you finger her last night?"
"Nah man! she would not let me but she did let me trampoline her..."